1. Field of the Invention
The present invention is related to an organic electroluminescent device and its manufacturing method. The present invention is related to an organic electroluminescent device and a manufacturing method comprising formation of an organic luminous layer by a wet method with the use of an organic luminescence ink, wherein the organic luminous layer formation material is dissolved in a solvent.
2. Description of the Related Art
An organic electroluminescent device has an organic luminous layer comprising organic luminescent material between two opposing electrodes. By means of applying electric current to the organic luminous layer between both electrodes, the organic luminous layer emits light. Organic luminescence layer thickness is important so that the organic luminous layer emits light efficiently. Organic luminescence layer thickness has to be about 100 nm. Even more particularly, for example, it is necessary to form a pattern of R (red), G (green), and B (blue) with high accuracy to make this a display panel.
Low molecular materials and polymeric material are examples of organic luminescent materials that can be used to form the organic luminous layer. Generally, as for low molecular materials, thin film is formed by vacuum evaporation. A mask with minute patterns is used at this time, and patterns are formed. In a manufacturing method with the use of vacuum processing such as vacuum evaporation, upsizing of a substrate can diminish the accuracy of the patterning. In addition, due to layering in vacuum, throughput is bad.
Thus the following method is tried recently. Macromolecular organic luminescent material is dissolved in a solvent, and coating slip is made. This coating slip is used, and thin film is formed by wet coating method. As wet coating methods to form thin film, spin coating method, bar coat method, extrusion coat method, dip coat method are exemplified.
It is difficult to form high minute pattern by a wet coating. In addition, it is difficult to paint so that three colors of the pattern of RGB are separated.
As for the formation method of thin film by a printing method, a separated pattern is formed more easily. Therefore, formation method of thin film by a printing method is more effective.
As for methods to print this organic luminescence ink, the following methods are exemplified: Offset printing (Japanese Patent Laid-Open No. 2001-93668 Official Gazette) to use rubber blanket having elasticity; relief printing method (Japanese Patent Laid-Open No. 2001-155858 Official Gazette) to use rubber printing plate and resin printing plate having elasticity; and ink jet method (Japanese Patent Laid-Open No. 2002-305077 Official Gazette).
Solubility of an organic luminescent material in a solvent can be low. When organic luminescence ink is made by means of dissolution in a solvent of these organic luminescent materials, concentration of organic luminescent material should be about 1%. Therefore, organic luminescence ink is the ink has a low viscosity.
In the case of formation of the organic luminous layer on a substrate by relief printing method, the organic luminescence ink having low viscosity of a concentration of about 1% is just transferred on a substrate from relief printing plate. In the case of the formation of the organic luminous layer on a substrate by ink jet method, organic luminescence ink discharged by ink jet nozzles of ink jet devices just fall into a substrate. It paints so that three color patterns of R, G and B are separated.
When organic luminescence ink of low viscosity is used, organic luminescence ink supplied on a substrate spreads. Therefore, organic luminescence inks of different luminescent colors are mixed. Color contamination occurs.
Thus, the following method has been proposed to control spreading of organic luminescence ink. Partition walls are made between first electrodes. Organic luminescence ink is printed onto first electrodes partitioned off by partition walls. Color contamination by spreading of inks can be prevented by the partition walls between first electrodes.
Partition walls are formed between pattern-shaped electrodes. Organic luminescence ink is used, and an organic luminous layer is formed. When a side surface of the partition wall is wettable to the organic luminescence ink, an organic luminescence layer thickness near the partition wall is bigger than the thickness of the middle of pixel after drying of organic luminescence ink. Therefore, luminance unevenness in a pixel occurs due to difference between organic luminescence layer thickness near the partition wall and organic luminescence layer thickness of the middle of pixel.
On the other hand, when side surfaces of the partition walls have organic luminescence ink repellency, organic luminescence layer thickness near the partition walls is thinner than that of the middle of pixel. In the organic luminous layer near partition walls, breakdown of the organic luminous layer occurs by electric field concentration due to the thickness being thin. In addition, luminance unevenness in a pixel occurs.
Profile explanatory drawing of a conventional organic electroluminescent device is shown in FIG. 1 (a). In addition, FIG. 1 (b), (c) are the extended figures of FIG. 1 (a). First electrode is patterned on substrate 1. Partition walls 7 are provided between patterns of first electrodes 2. Hole transport layer 3 is formed on the first electrodes 2 partitioned off by means of partition walls 7. Red (R) organic luminous layer 41, green (G) organic luminous layer 42 and blue (B) organic luminous layer 43 are formed on hole transport layer 3, respectively. And the second electrode 5 is formed on organic luminous layer (41, 42, 43). An organic luminous layer or a hole transport layer is formed by a wet method such as a printing method or coating method.
FIG. 1 (b) is a figure to show the case that side surfaces of partition walls are ink wettable. Organic luminescence layer thickness near partition walls is thicker than thickness of the middle of pixel. Due to nonuniformity of thickness of an organic luminous layer and a hole transport layer near partition walls, luminance unevenness in a pixel is caused. L is a light emitting area.
In addition, FIG. 1 (c) is a figure to show the case that side surfaces of partition walls have ink repellency. Organic luminescence layer thickness near partition wall is thinner than organic luminescence layer thickness in the middle of pixel. Due to nonuniformity of thickness of an organic luminous layer and a hole transport layer, luminance unevenness in a pixel is caused. Even more particularly, due to electric field concentration in a thin part of thickness near partition walls, breakdown of the organic luminous layer is caused.
Due to a difference of affinity of partition wall to organic luminescence ink, organic luminescence layer thickness near partition walls changes. Due to change of this organic luminescence layer thickness, luminance unevenness in a pixel and breakdown by electric field concentration occur. It is necessary to select formation material of organic luminescence ink or partition wall appropriately to prevent luminance unevenness in a pixel and a breakdown by electric field concentration. The method of how additive controlling affinity between ink and partition wall is mixed with organic luminescence ink or partition wall material is suggested. These additives may have harmful effect on luminescence property of an organic electroluminescent display unit. Therefore, the selection of additive is very difficult.
According to the present invention, organic luminescence ink comprising organic luminescent material is used, and an organic luminous layer is formed in pixel electrodes partitioned off by partition walls. The present invention provides an organic electroluminescent display unit without luminance unevenness in a pixel and dielectric breakdown by electric field concentration due to change of organic luminescence layer thickness near partition walls.